Count business days excluding holidays
Company: Airtable
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: medium
Interview Round: Onsite
## Problem: Count Working Days Between Two Dates
Given a start date, an end date (inclusive), and a set of holiday dates, compute the number of **working days**.
A working day is a date that:
- is **not** Saturday or Sunday, and
- is **not** in the provided holiday set.
### Input
- `startDate`: string in `YYYY-MM-DD`
- `endDate`: string in `YYYY-MM-DD` (guaranteed `endDate >= startDate`)
- `holidays`: list of distinct date strings in `YYYY-MM-DD`
### Output
- An integer: number of working days in `[startDate, endDate]`.
### Example
- `startDate = 2026-12-24`
- `endDate = 2026-12-28`
- `holidays = [2026-12-25]`
Dates:
- 12/24 Thu (work)
- 12/25 Fri (holiday, not work)
- 12/26 Sat (weekend)
- 12/27 Sun (weekend)
- 12/28 Mon (work)
Output: `2`
### Constraints (typical)
- Date range length up to \(10^6\) days for a naive iteration; design an approach appropriate to the stated constraints.
- Holiday count up to \(10^5\).
Quick Answer: This question evaluates proficiency in date and calendar arithmetic, handling set-based exclusions (holiday lists), and reasoning about algorithmic efficiency for counting working days across potentially large ranges.